The present invention relates to a short wavelength optical disk head that is compatibly capable of recording on and reproducing from a long wavelength optical disk with use of short wavelength.
An optical disk can record information such as images, sounds and data on it at a high density or reproduce any of them from it, so that the information can be recorded and reproduced at high S/N ratio.
Light sources for prior optical disk heads, which are of semiconductor lasers, generate infrared lights of around 780 and 830 nm wavelengths. Recording density of the optical disk is made high as the wavelength of the light source is short. In recent years, therefore, in order to increase the recording density several times that of the infrared light, it has been attempted to use as the light source for an optical disk, second harmonics of the semiconductor laser or of the solid state laser and short wavelength lasers of red, yellow, green, blue, violet, ultraviolet having wavelength of shorter than 700 nm. The short wavelength lasers employ semiconductor crystals of II-VI compounds which provide wide bandwidths. If the wavelength of the light source is halved, for example, the diameter of the focused light spot is halved. The area becomes one fourth. As a result, the recording density of the optical disk is multiplied by four. There are known reports about the optical disk, such as Masabumi Mori et al, "Hikari Disk" (Optical Disk), page 2, May 10, 1988, Ohm Publishing Co.; and G. Bouwhuis et al, "Principles of Optical Disc Systems," pp. 2-3, 1985, Adam Hilger Ltd.
As such, the very high density optical disk for use with the light source of shorter wavelength has to naturally have smaller size of minimum units, which is called the pits, for recording and reproducing signals with decreasing the wavelength. Also, the track pitch has to be made narrower with decreasing the wavelength. If the second harmonics of such a solid state laser as Nd:YAG or Nd:YVO.sub.4 laser is used to obtain wavelength of 530 nm, as an example, then the track pitch becomes 0.9 .mu.m and the pit size is 0.4 .mu.m. On the other hand, the usual compact disks (CD) having used infrared lights of around 780 and 830 nm wavelengths are 1.6 .mu.m in the track pitch and around 0.7 .mu.m in the minimum pit size.
FIG. 2 is a graph illustrating the light intensities of the light spots formed on the optical disk. In the figure are indicated an optical disk 7, pits 13 and 14 on the disk 7, and light intensity curves 11 and 12 of the light spots formed on disk 7 by the laser beam. The solid curve and line in the figure indicate the light intensity of the light spot and the pit for a short wavelength of 700 .mu.m or shorter. The dotted curve and line are the light intensity of the light spot and the pit for a wavelength of longer than 700 .mu.m.
The optical disk for the shorter wavelength has to have resolution increased by narrower light spot 11 enough to provide high modulation depth for signal reproduction. The reason is that the disk is smaller in the size of pit 13. The optical disk for the longer wavelength, on the other hand, should played back with larger spot 12 to provide high modulation as the disk is larger in the size of pit 14. In other words, if the depth of the pit is around .lambda./4 where .lambda. is the wavelength of the laser beam, an optimum diameter of the light spot is around two times that of the pit, that is, an optimum diameter of the pit is around one second of that of the light spot since the light of the light spot reflected from the inside of the pit deviates by phase of .pi. from the light reflected from the outside of the pit.
Therefore, if the newly developed short wave optical disk driver plays back the convertional long wavelength optical disk (CD) available on the market heretofore, the diffraction effect of the light is reduced with the long wavelength CD played back by the smaller light spot 11 as it is, since pit 14 is larger. This results in decreased modulation depth for presence and absence of the pit. This in turn results in decreased signal-to-noise ratio of the reproduced signal. If the long wavelength optical disk driver plays back the short wavelength optical disk, on the contrary, it also reduces the diffraction effect of the light with the short wavelength optical disk played back by the larger light spot 12 as it is, since pit 13 is smaller. This also results in decreased modulation depth for presence and absence of the pit. This again results in decreased signal-to-noise ratio of the reproduced signal.
Types of the optical disks include a rewritable type, and a write once type, in addition to reproduction only type such as of compact disk. That is, the magneto-optical disk should have data erased once before newly writing the data since it cannot simultaneously erase and write the data. There however was recently proposed an overwriting technique in which data can be rewritten at a time. In connection with the material for the optical disk, a phase change type disk was proposed in which its crystal state can be changed to amorphous state, and vice versa. There also is used a write once type optical disk that has a partial area in which data can be written with laser beam irradiated to make pits.
As pointed out above, it is anticipated that the optical disk will be made denser year after year with use of shorter wavelength of the light source. However, this involves a problem of compatibility of new very high density optical disks developed in future with the usual optical disks at present. In other words, it is a serious problem to solve of whether the very high density optical disk driver having the short wavelength light source can play back the conventional compact disk (CD) or laser disk. If the compatibility is ignored in that sense, this makes it impossible to play back numbers of usual optical disks having already come into wide use in homes and offices. This is an act of treachery for customers and consumers. It is an important issue imposed on not only the reproduction only optical disk, but also the rewritable magneto-optical disk, phase change type disk, and write once type optical disk.
In order to solve that problem, the Japanese Patent Application Laid-Opens 64-3833 and the 02-252137 disclosed an optical head having an aperture diaphragm for restricting diameter of a light beam placed in front of an object lens for focusing the light beam onto an optical recording medium. The aperture diaphragm has the disadvantage: the aperture diameter of the aperture diaphragm made smaller causes shading for the light. The shading decreases the light amount coming to the optical recording medium. This also results in decreased light amount coming to a photodetector, which in turn results in decreased signal intensity.